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Maffia et al. Carbon Footprints 2026, 5, 7 Page 15 of 19

Figure 5. Correlation matrix among the Carbon Footprint (CFP, kg CO 2 eq ton ) and the main chemical, microbiological and enzymatic soil
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parameters. Variables included: pH (H 2 O, 1:2.5), EC (Electrical Conductivity, dS m ), WC (Water Content, %), WSP (Water Soluble
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Phenols, mg TAE g d.w.), TOC (Total Organic Carbon, %), TN (Total Nitrogen, %), C/N (Carbon to Nitrogen ratio, dimensionless), SOM
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(Soil Organic Matter, %), HC (Humified Carbon, %), FC (Fraction of Carbon easily decomposable, %), HC/FC (Humified Carbon to
Fraction of Carbon ratio, %), MBC (Microbial Biomass Carbon, µg C g ), CEC [Cation Exchange Capacity, cmol(+) kg ], FUNGI (Fungi, %
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of total microbial biomass), BACT (Bacteria, % of total microbial biomass), ACT (Actinomycetes, % of total microbial biomass), FBC
(Fungal Biomass Carbon, µg C g ), BBC (Bacterial Biomass Carbon, µg C g ), ABC (Actinomycetes Biomass Carbon, µg C g ), FDA
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(Fluorescein Diacetate hydrolase activity, µg fluorescein g h ), and DHA (Dehydrogenase activity, µg TPF g h ). The scale ranges from -1
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(strong negative correlation, red) to +1 (strong positive correlation, green).
substantial gains in humification observed under compost and vermicompost suggest that these amendments
can meaningfully contribute to meeting this global target, particularly in vulnerable Mediterranean soils.
CONCLUSION
This study demonstrates that organic amendments derived from agro-industrial wastes have strong potential
to improve soil fertility, enhance humification, and contribute to climate change mitigation. Compost and
vermicompost emerged as the most effective strategies, showing the highest humification indices (HA/FA,
HC/FC, HR%, HD%) and fostering the formation of stable carbon pools. These treatments also promoted
balanced microbial communities, where fungi, bacteria, and actinomycetes acted synergistically to transform
organic inputs into persistent HSs. Digestate played an intermediate role, increasing TOC and fungal
biomass but with lower humification efficiency, suggesting that its contribution is more relevant for
short-term nutrient recycling than for long-term carbon sequestration. SBO significantly influenced
microbial composition by stimulating fungal dominance and acidifying the soil; however, its environmental
footprint, largely due to sulfur processing, limits its suitability as a climate-smart amendment. Life cycle
assessment further confirmed the advantages of compost and vermicompost, which showed the lowest CFPs
compared with digestate and SBO. By combining agronomic, biochemical, microbial, and environmental
evidence, this work highlights how waste-derived fertilizers can replace or reduce the use of synthetic inputs,
contributing simultaneously to soil quality, resource efficiency, and GHG mitigation.